| modified from: http://www.eng.fsu.edu/~tsai/raid/raid.html | |
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RAID 0: Striped Disk Array without Fault Tolerance  RAID Level 0 requires a
minimum of 2 drives to implement |
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Characteristics/Advantages RAID 0 implements a striped
disk array, the data is broken down into blocks and each block is
written to a separate disk drive I/O performance is greatly improved by spreading the I/O load across
many channels and drives Best performance is achieved when data is striped across multiple
controllers with only one drive per controller No parity calculation overhead is involved Very simple design Easy to implement |
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Disadvantages Not a "True" RAID
because it is NOT fault-tolerant The failure of just one drive will result in all data in an array
being lost Should never be used in mission critical environments
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RAID 1: Mirroring and Duplexing  For Highest performance, the
controller must be able to perform two concurrent separate Reads per
mirrored pair or two duplicate Writes per mirrored pair. RAID Level 1 requires a minimum of 2 drives to implement |
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Characteristics/Advantages One Write or two Reads
possible per mirrored pair Twice the Read transaction rate of single disks, same Write
transaction rate as single disks 100% redundancy of data means no rebuild is necessary in case of a
disk failure, just a copy to the replacement disk Transfer rate per block is equal to that of a single disk Under certain circumstances, RAID 1 can sustain multiple simultaneous
drive failures Simplest RAID storage subsystem design |
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Disadvantages Highest disk overhead of all
RAID types (100%) - inefficient Typically the RAID function is done by system software, loading the
CPU/Server and possibly degrading throughput at high activity levels.
Hardware implementation is strongly recommended May not support hot swap of failed disk when implemented in
"software"
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RAID
2: Hamming Code ECC Each bit of data word is
written to a data disk drive (4 in this example: 0 to 3). Each data word
has its Hamming Code ECC word recorded on the ECC disks. On Read, the
ECC code verifies correct data or corrects single disk errors. |
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Characteristics/Advantages "On the fly" data
error correction Extremely high data transfer rates possible The higher the data transfer rate required, the better the ratio of
data disks to ECC disks Relatively simple controller design compared to RAID levels 3,4 &
5 |
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Disadvantages Very high ratio of ECC disks
to data disks with smaller word sizes - inefficient Entry level cost very high - requires very high transfer rate
requirement to justify Transaction rate is equal to that of a single disk at best (with
spindle synchronization) No commercial implementations exist / not commercially viable |
RAID 3: Parallel transfer with parity  The data block is subdivided
("striped") and written on the data disks. Stripe parity is
generated on Writes, recorded on the parity disk and checked on Reads. RAID Level 3 requires a minimum of 3 drives to implement |
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Characteristics/Advantages Very high Read data transfer
rate Very high Write data transfer rate Disk failure has an insignificant impact on throughput Low ratio of ECC (Parity) disks to data disks means high efficiency |
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Disadvantages Transaction rate equal to that
of a single disk drive at best (if spindles are synchronized) Controller design is fairly complex Very difficult and resource intensive to do as a "software"
RAID
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RAID 4: Independent Data disks with shared Parity disk  Each entire block is written
onto a data disk. Parity for same rank blocks is generated on Writes,
recorded on the parity disk and checked on Reads. RAID Level 4 requires a minimum of 3 drives to implement |
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Characteristics/Advantages Very high Read data
transaction rate Low ratio of ECC (Parity) disks to data disks means high efficiency High aggregate Read transfer rate |
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Disadvantages Quite complex controller
design Worst Write transaction rate and Write aggregate transfer rate Difficult and inefficient data rebuild in the event of disk failure Block Read transfer rate equal to that of a single disk |
RAID 5: Independent Data disks with distributed parity blocks  Each entire data block is
written on a data disk; parity for blocks in the same rank is generated
on Writes, recorded in a distributed location and checked on Reads. RAID Level 5 requires a minimum of 3 drives to implement |
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Characteristics/Advantages Highest Read data transaction
rate Medium Write data transaction rate Low ratio of ECC (Parity) disks to data disks means high efficiency Good aggregate transfer rate |
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Disadvantages Disk failure has a medium
impact on throughput Most complex controller design Difficult to rebuild in the event of a disk failure (as compared to
RAID level 1) Individual block data transfer rate same as single disk
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RAID
6: Independent Data disks with two independent distributed parity
schemes |
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Characteristics/Advantages RAID 6 is essentially an
extension of RAID level 5 which allows for additional fault tolerance by
using a second independent distributed parity scheme (two-dimensional
parity) Data is striped on a block level across a set of drives, just like in
RAID 5, and a second set of parity is calculated and written across all
the drives; RAID 6 provides for an extremely high data fault tolerance
and can sustain multiple simultaneous drive failures Perfect solution for mission critical applications |
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Disadvantages Very complex controller design Controller overhead to compute parity addresses is extremely high Very poor write performance Requires N+2 drives to implement because of two-dimensional parity
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RAID 7: Optimized Asynchrony for High I/O Rates as well as High Data Transfer Rates  Architectural Features:
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Characteristics/Advantages Overall write performance is
25% to 90% better than single spindle performance and 1.5 to 6 times
better than other array levels Host interfaces are scalable for connectivity or increased host
transfer bandwidth Small reads in multi user environment have very high cache hit rate
resulting in near zero access times Write performance improves with an increase in the number of drives
in the array Access times decrease with each increase in the number of actuators
in the array No extra data transfers required for parity manipulation RAID 7 is a registered
trademark of Storage Computer Corporation. |
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RAID 10: Very High Reliability combined with High Performance  RAID Level 10 requires a
minimum of 4 drives to implement |
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Characteristics/Advantages RAID 10 is implemented as a
striped array whose segments are RAID 1 arrays RAID 10 has the same fault tolerance as RAID level 1 RAID 10 has the same overhead for fault-tolerance as mirroring alone High I/O rates are achieved by striping RAID 1 segments Under certain circumstances, RAID 10 array can sustain multiple
simultaneous drive failures Excellent solution for sites that would have otherwise gone with RAID
1 but need some additional performance boost |
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RAID 53: High I/O Rates and Data Transfer Performance  RAID Level 53 requires a
minimum of 5 drives to implement |
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Characteristics/Advantages RAID 53 should really be
called "RAID 03" because it is implemented as a striped (RAID
level 0) array whose segments are RAID 3 arrays RAID 53 has the same fault tolerance as RAID 3 as well as the same
fault tolerance overhead High data transfer rates are achieved thanks to its RAID 3 array
segments High I/O rates for small requests are achieved thanks to its RAID 0
striping Maybe a good solution for sites who would have otherwise gone with
RAID 3 but need some additional performance boost |
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Disadvantages Very expensive to implement All disk spindles must be synchronized, which limits the choice of
drives Byte striping results in poor utilization of formatted capacity |
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RAID 0+1: High Data Transfer Performance  RAID Level 0+1 requires a
minimum of 4 drives to implement |
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Characteristics/Advantages RAID 0+1 is implemented as a
mirrored array whose segments are RAID 0 arrays RAID 0+1 has the same fault tolerance as RAID level 5 RAID 0+1 has the same overhead for fault-tolerance as mirroring alone High I/O rates are achieved thanks to multiple stripe segments Excellent solution for sites that need high performance but are not
concerned with achieving maximum reliability |
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Disadvantages RAID 0+1 is NOT to be confused
with RAID 10. A single drive failure will cause the whole array to
become, in essence, a RAID Level 0 array Very expensive / High overhead All drives must move in parallel to proper track lowering sustained
performance Very limited scalability at a very high inherent cost
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